The present invention relates to magnetohydrodynamic (hereinafter abbreviated "MHD") generators, and more particularly to such generators that employ either the Hall or diagonal method of connection.
In general terms, MHD generators produce electric power by movement of electrically conductive fluid relative to a magnetic field. The fluid employed is usually an electrically conductive gas from a high temperature, high pressure source. From the source, the fluid flows through the generator and, by virtue of its movement relative to the magnetic field, induces an electromotive force between opposed electrodes within the generator. The gas may exhaust to a sink, which may simply be the atmosphere; or, in more sophisticated systems, the gas may exhaust to a recovery system including pumping means for returning the gas to the source.
Several different gases may be used, for example, the gas may simply be air, or may comprise inert gases, such as helium or argon. To promote electrical conductivity, the gases are heated to high temperature and may be seeded with a substance that ionizes readily at the operating temperature of the generator. For seeding purposes, sodium, potassium, cesium or an alkali metal vapor may be used. Regardless of the gas used and the manner of seeding, the resulting gases comprise a mixture of electrons, positive ions and neutral atoms which, for convenience, may be termed "plasma."
In the conventional MHD generator, the plasma flows through a magnetic field, which is directed perpendicular to the direction of plasma flow. The movement of the electrically conductive plasma relative to the field produces an E.M.F. that is normal both to the direction of flow of the plasma and the magnetic field, the current flowing transversely of the field between opposed electrodes at the sides of the generator. In such a generator, a separation of positive and negative electrical charges occurs along the length of the plasma stream, producing a potential gradient, known as the "Hall potential," which promotes longitudinal circulation of current internally of the generator. In a conventional MHD generator, such longitudinal currents cause energy losses which are detrimental to the operation of the generator and various schemes have been devised to prevent their formation. It is possible, however, to build an MHD generator that takes advantage of the Hall potential, as in the so-called "Hall current generator" and the so-called "diagonal generator."
The Hall current generator comprises a duct and a magnetic field normal to the axis of the duct. Movement of plasma through the duct and the field induces an electromotive force between numerous pairs of opposed discrete electrodes that are interconnected to accommodate circulation of current transversely of both the magnetic field and the direction of plasma flow. The terminal electrodes, i.e., the first and last electrodes along the length of the duct, are connected to an external load, making possible circulation of Hall current longitudinally through the plasma and the load circuit. Oppositely disposed electrodes intermediate the terminal electrodes are interconnected to provide the aforementioned transverse circulation of current. The arrangement of elements is quite simple and effective.
The diagonally connected generator is somewhat more complex, but more efficient than the Hall generator. Its operation may be explained as follows: Movement of the electrically conductive plasma past each pair of opposed discrete electrodes generates a potential gradient in the plasma therebetween. Assuming that the velocity of the plasma and the magnetic field strength are constant the length of the duct, then the potential difference established transverse of the duct between any given pair of opposed electrodes is substantially constant; however, because of the Hall potential existing longitudinally of the duct, the mean potential of the last pair of electrodes is at a more positive level than that of the first pair of electrodes. Thus, the mean potential of any given pair of opposed electrodes is more positive than the mean potential of opposed electrodes that are upstream thereof.
By suitable spacing of the electrodes along the duct, it is possible, under ideal circumstances to make the potential of staggered, opposed electrodes the same. Since these electrodes are at the same potential, they may be electrically interconnected. A structure such as this with the diagonally opposed electrodes interconnected is described in U.S. Pat. No. 3,148,291, which issued Sept. 8, 1964. In this structure, it will be noted that the power generated by each pair of opposed electrodes is added to the power of all other pairs of electrodes and supplied to the terminal electrodes. Thus, a generator of the type described not only is able to deliver power at high voltage, but substantial amounts of power to a common load.
One problem that arises with the Hall generator and also the "diagonally connected MHD generator" is that there is a tendency for the current flowing in the gas to flow preferentially to some electrodes and not to others due, for example, to random variations in their surface properties or structure. This can lead to concentrations of Hall voltage between certain electrodes and lead to damaging breakdown--especially on the anode wall.